It has become common to dynamically display aerial images, such as aerial photographs, in response to user interactions. For example, a user visiting an aerial imaging web site may type a postal address in order to display an aerial image centered on that address at a medium level of magnification. The user may perform additional interactions to zoom in or out, or to pan in various compass directions. In some cases, the displayed image is overlaid or otherwise composited with various additional features, including roads, political boundaries, geographic names, or real property parcels.
Such dynamic display of aerial images is typically performed using image data captured and processed as follows. First, a number of photographs are taken of the relevant geographic region from an aerial platform, such as an airplane or a satellite. Each photograph is tagged with polar or other spherical coordinates—e.g., latitude-longitude pairs—identifying the area shown. The photographs are then concatenated together on the surface of a 3-dimensional solid representing the Earth, such as a sphere or a spheroid. Because the images are generally ultimately displayed on a flat surface, the coordinates with which it is indexed on the surface of a 3-dimensional solid must be transformed into coordinates in a single plane before being incorporated in a displayed image. This transformation is commonly accomplished by (1) performing a cylindrical projection in which points on the surface of the sphere or spheroid representing the Earth are projected onto the lateral surface of a cylinder whose central axis coincides with a polar axis of the sphere or spheroid that corresponds to the Earth's polar axis and that intersects the sphere or spheroid at the equator (i.e., a cylindrical projection in which the standard parallel is the equator), (2) then unrolling the lateral surface of the cylinder to produce a plane, sometimes called a “master image.” This master image is then divided into a 2-dimensional array of rectangular portions, called “tiles.” When a dynamic aerial image request is received for a particular location and magnification level, one or more tiles about the location are retrieved, joined, and trimmed in a manner that satisfies the request, and returned.
Unfortunately, the conventional approach described above has important disadvantages. First, it tends to distort the horizontal scale of tiles near extreme latitudes of the covered region, i.e., portions of the region nearest a pole. FIG. 1 is a map diagram showing the distortion effect suffered by conventional approaches at extreme latitudes. Tiles 110, 120, 130, 140, and 150 are tiles from the Northern hemisphere that progress in a direction from South to North. The width of these tiles is constant in terms of longitude. For example, each tile may have a longitudinal width of 0.01° longitude. However, as the tiles move closer to the pole, that is, as they move North, their width in distance declines. For example, it can be seen that the width 123 in distance at the top of tile 120 is smaller than the corresponding width 113 at the top of tile 110. Indeed, when tile 150 is reached, the width in distance is substantially reduced to width 153. In performing the cylindrical projection using the equator as the standard parallel, the conventional approach has the effect of expanding the width of the image for each tile to a degree that increases with proximity to a pole. For example, it can be seen that the width of the image for tile 150 is increased dramatically from width 153 to width 156, a horizontal magnification of about 200% relative to images at the standard parallel latitude. As a result, a visual feature shown in tile 140 would appear twice as wide (but no taller) than a feature of the same size shown in tile 110. This distortion effect can significantly impair the recognizability of aerial imagery, and the ability to form accurate spatial perceptions from it.
The conventional approach also has the disadvantage that it tends to magnify conversion errors from latitude/longitude coordinates commonly used to refer to locations into planar coordinates in the master image. FIG. 2 is a map diagram showing the location and accuracy disadvantage suffered by the conventional approaches. The diagram shows a master image 200 that has been divided into tiles using the conventional approach, beginning from an origin of 122.00° West, 47.50° North. The tiles are each intended to represent an area 0.01° wide and 0.01° tall. The conventional approach proceeds by converting this spherical width and height into distance. In particular, it converts the 0.01° width to 0.4665 miles, and the 0.01° height to 0.690 miles. It moves to the right by this width distance and down by this height distance from the origin to define the first tile 210. It moves to the right by the width distance again to define the tile 230 immediately to the East, and down again by the height distance to define the tile 240 immediately to the South, proceeding this way until the master image has been completely divided into tiles. The borders between adjacent tiles defined in this manner are shown in the diagram by broken lines. For example, tile 210 begins at the origin and is bordered by the first broken line segments to the right and down, respectively. It is typical for a certain amount of error to be present in the converted distances, introduced by errors in measurement, errors in calculation, and/or rounding error. While the inaccuracy of area covered by tiles resulting from this error can be small near the origin—as reflected by the relatively small difference between tile 210 and corresponding intended tile area 260, this inaccuracy worsens at greater distances from the origin. For example, it can be seen that tile 220 is significantly dislocated from its intended tile area 270, to such an extent that a displayed aerial image that relied upon tile 220 would not contain the correct information, would show visual features of the image in the wrong positions, and would accordingly poorly coordinate the positions of the visual features of the aerial images with composited external visual information. As an example, a house visual feature 272 is included in the master image at the position shown. While its position places it in the center of intended tile area 270, it is actually included at the lower left hand corner of corresponding tile 220. The house that is the subject of this visual feature is at the center of a real estate parcel 271. Because the conventional approach believes that the tile 220 corresponds to the area of intended tile area 270, however, it will composite the parcel boundary visual information 221—i.e., the outline of the parcel—in the center of tile 220. Rather than surrounding the house visual feature as it should, the displayed parcel boundary excludes the house. This is not helpful to a user who wishes to understand how buildings are situated on a parcel, how the parcel is situated relative to visual features of the surrounding neighborhood, etc.
Accordingly, a process for generating dynamic aerial images that provide optimized aerial image portions for accurate compositing with other visual information would have significant utility.